Method for manufacturing solid state image sensing device formed of charge coupled devices on side surfaces of trenches

ABSTRACT

A solid state image sensing device comprises photoelectric converting portions (8) and charge coupled portions. A plurality of parallel trenches (2) are formed on a main surface of a semiconductor substrate (1). Photoelectric converting portions are on the surfaces of the semiconductor substrate on both sides of each of the trenches. Charge transfer portions corresponding to the photoelectric converting portions are independently formed on the side surfaces of the trenches. Insulating and isolating regions (15, 29, 30) are formed on the bottom portions of the trenches. By providing two independent charge transfer portions in one trench, the area occupied by the charge transfer portions can be reduced.

This application is a division of application Ser. No. 0/446,261, filedDec. 5, 1989 now U.S. Pat. No. 5,029,321.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement of highly integratedstructure of charge coupled devices and to an improvement of highlyintegrated structure of solid state sensing devices employing chargecoupled devices. The present invention is also related to a device.

2. Description of the Background Art

A solid state image sensing device is used in a TV camera, a videocamera and so on, which converts optical images into electrical signalsto pickup the images. A charge transfer device is used as an apparatusfor transferring the converted electrical signals in a prescribedoperation.

A charge transfer device transfers charges stored on a surface of or ina semiconductor successively to a prescribed direction along thesurface. The charge transfer devices are applied to solid state imagesensing devices, memories and so on. There are two types of chargetransfer devices having different types of transfer electrodes. One typeof the charge transfer device has a unitary transfer gate extendingcontinuously along the direction of the charge transfer. The chargetransfer device of this type is disclosed in, for example, U.S. Pat. No.4,760,273. Another type of charge transfer device has a plurality oftransfer electrodes arranged along the direction of charge transfer. Thecharge transfer device of this type is called a charge coupled device(CCD), and the present invention is related to the CCD. When a pluralityof MOS capacitors are arranged close to each other so that the depletionregions of the capacitors are overlapped with each other and potentialwells are coupled with each other, externally applied charges aretransferred as charge packets from positions having higher potential topositions having lower potential. The CCD utilizes the above describedphenomenon. More specifically, when a clock voltage is applied to gateelectrodes of a number of charge coupled MOS capacitors, charge packetsare successively transferred along a channel formed on a surface of asemiconductor substrate.

A planar type CCD has been known in which MOS capacitors are arranged ona main surface of a semiconductor substrate along the direction oftransfer to form a charge transfer region. Now, in a CCD employed in asolid state image sensing device, a photosensitive region and the chargetransfer region of the CCD are arranged in two dimensions on the mainsurface of the semiconductor substrate. In such a solid state imagesensing device, the degree of integration and opening ratio must beincreased to improve resolution and sensitivity. The opening ratio meansthe proportion of the area of the photosensitive region occupying thesurface of the substrate. The degree of integration means the number ofelements included in a unit area. However, as the degree of integrationbecomes higher, the proportion of the surface area occupied by the CCDis increased and the opening ratio is lowered. Consequently, thesensitivity is lowered, preventing the improvement of resolution.

In view of the foregoing, a CCD having a structure enabling higherdegree of integration has been proposed. Such a CCD is disclosed in, forexample, Japanese Patent Laying-Open Gazette NO. 290175/1987. FIG. 12 isa perspective cross sectional view showing the structure of the CCDshown in the above mentioned gazette and FIG. 13 is a plan view of thestructure. A plurality of trenches 2b, 2c, 2d and 2e extending parallelto each other are formed on a main surface of a p type siliconsubstrate. An insulating film 3 formed of silicon oxide is formed overthe surface of the p type silicon substrate 1 and inner surfaces of thetrenches 2b to 2e. Electrodes 4b, 4c, 4d and 4e elongated along thetrenches are formed in the trenches 2b to 2e. Channel regions 5 of ntype impurity regions are formed in contact with the inner surfaces ofthe trenches 2b to 2e in the p type silicon substrate 1. Each of thechannel region 5 is divided into four regions I to IV. The regions I andII are opposed to an electrode 4a with an insulating film 3 interposedtherebetween. The region I is an n⁻ region and the region II is an nregion. The regions III and IV are formed opposed to the electrode 4bwith the insulating film 3 interposed therebetween. The region III is ann⁻ region and the region IV is an n region. A portion of the region IIis in contact with a portion of the region III. The electrodes 4a, 4cand 4e are connected to source φ1, while the electrodes 4b and 4d areconnected to a clock pulse source φ2. Clock voltages having differentphases are applied from the clock pulse sources φ1 and φ2. In operation,the charges stored in the channel regions 5 are successively transferredin the direction of the arrow 6 in accordance with the height of thepotential wells formed by the high level voltage VH and the low levelvoltage VL applied from the clock pulse sources φ1 and φ2.

As described above, in the prior art, a trench is formed on a mainsurface of a substrate, transfer electrodes are formed therein and achannel region is formed on a sidewall of the trench in order to realizethe fine structure of a CCD. The fine structure is realized by reducinghorizontal area occupied by the charge transfer region formed on themain surface of the silicon substrate.

An example of the prior art will be described with reference to FIGS. 14and 15 in which the degree of integration is increased to enhance theresolution in a solid state image sensing apparatus employing CCDs. Thisexample is disclosed in, for example, Japanese Patent Laying-OpenGazette No. 51254/1987. Referring to FIGS. 14 and 15, the solid stateimage sensing device comprises an arrangement of photosensitive regions8 and vertical charge transfer regions 9 formed on a surface of asilicon substrate 7. Each of the photosensitive region 8 comprises a pwell region 10 provided on the n type silicon substrate 7 and an n typeimpurity region 11 formed on the surface of the region 10, therebyproviding a pn junction. Photoelectric charges excited by light enteringthe photosensitive region 8 are stored in the pn junction region. Aplurality of electrodes are arranged adjacent to the photosensitiveregions 8 in the direction of the column of FIG. 14, thereby providingCCDs of the vertical charge transfer regions 9. The electrodes 4 areconnected to each other in the low direction by means of leads 12. Theelectrode 4 is formed in a trench 2 provided on the main surface of thesilicon substrate 7 with an insulating film 3 interposed therebetween. Aplurality of electrodes are aligned in the column direction, and endportions of alternate electrodes are laid over upper portions of theremaining ones of the electrodes. Every other ones of the electrodes 4have gate electrode portions 13 for transferring the photoelectriccharges stored in the photosensitive regions 8 to the n channel regions5 of the CCDs. An n type impurity region 5 forming the n channel regionis formed in the inner surface region of the trench 2.

As described above, the solid state image sensing device of the priorart comprises a plurality of photosensitive regions 8 arranged in amatrix and CCDs (vertical charge transfer regions 9) for transferringthe photoelectric charges generated in the photosensitive regions 8 inthe vertical direction, each of which connected to each of thephotosensitive regions 8. The CCDs are alternately aligned with thelines of the plurality of photosensitive regions 8 arranged in thecolumn direction. By providing the electrode 4 and the n channel region5 for charge transfer of the CCD in the trench 2, the horizontal areaoccupied by the vertical charge transfer region 9 is reduced, therebyimproving the opening ratio of the photosensitive region 8.

The operation of the solid state image sensing device of the abovedescribed prior art will be described in the following. The lightentering the photosensitive region 8 excites photoelectric charges inthis region, and the photoelectric charges are stored in the pn junctionregion. When a positive pulse is applied to the electrode 4 having thegate electrode portion 13, an n channel (not shown) is formed on thesurface of the silicon substrate below the gate electrode portion 13, sothat the charges stored in the photosensitive region 8 are transferredto the n channel region 5 of the CCD. Thereafter, a driving clock signalis applied to the plurality of electrodes 4, whereby the charges storedin the n channel regions 5 of the CCD are transferred in the vertical(column) direction through the n channel region 5. When all the chargesof 1 pixel are transferred in the vertical direction, the charges of 1pixel are transferred to the CCD for horizontal transfer (not shown) onthe end portion of CCD for vertical transfer and then transferred in thehorizontal (row) direction. When the charges transferred to the CCD forhorizontal transfer are transferred in the horizontal direction to beoutputted, then the driving clock signal is again applied to theplurality of electrodes 4. Therefore, the charges are transferred in thevertical direction by 1 pixel, the charges of 1 pixel on the end portionof the CCD for vertical transfer are transferred to the CCD forhorizontal transfer, and they are transferred in the horizontaldirection by the CCD for horizontal transfer to be outputted. By therepetition of the above described operation, output signalscorresponding to the light incidental to the photosensitive regions 8are provided.

AS described above, in the conventional CCD or in the solid state imagesensing device employing CCDs, trenches are formed in the surface of thesilicon substrate, and the CCDs are formed in the trenches in order torealize highly integrated structure or fine structure and to reduce thehorizontal area of occupation. However, these structures are onlypartial modifications of the conventional planar type structure, whichis formed in the trench in the substrate surface. Therefore, in theabove described solid state image sensing device, the horizontalarrangement of the photosensitive regions and the charge transferregions is the same as in the conventional planar type structure.Therefore, the reduction of the horizontal area occupied by the chargetransfer regions on the surface of the substrate and the improvement ofthe degree of integration are limited.

SUMMARY OF THE INVENTION

An object of the present invention is to reduce horizontal area occupiedby CCDs formed on a semiconductor substrate.

Another object of the present invention is to improve the opening ratioof a solid state image sensing device.

A further object of the present invention is to miniaturize thestructure of the solid state image sensing device.

A still further object of the present invention is to provide a methodfor manufacturing a solid state image sensing device having a finestructure.

In accordance with an aspect of the present invention, the chargecoupled device is formed on a semiconductor substrate having a trench.On one of the two side surfaces of the trench formed are impurityregions of a second conductivity type and charge transfer electrodes onthe surface of the impurity regions with insulating film posedtherebetween. The plurality of charge transfer electrodes are arrangedalong a longitudinal direction of the trench. In the same manner,impurity regions, insulating films and charge transfer electrodes areformed on the other one of the sidewalls of the trench. Insulating andisolating means are formed at the bottom of the trench for insulatingand isolating impurity regions on both sidewalls from each other.

Namely, two charge coupled devices independent from each other areformed on both side surfaces of a trench formed in the surface of thesemiconductor substrate. Therefore, compared with the prior art devicein which one charge transfer device is formed in one trench, the degreeof integration of the device can be increased.

In another aspect of the present invention, a solid state image sensingdevice comprises a semiconductor substrate having a trench on the mainsurface thereof. On a pair of main surface regions of the semiconductorsubstrate near tee trench, a first line of photoelectric convertingelements and a second line of photoelectric converting elements arearranged. First charge transfer elements and second charge transferelements corresponding to the first and second lines of photoelectricconverting elements are separately formed on the two side surfaces ofthe trench. The first and second charge transfer elements areelectrically insulated and isolated from each other by insulating andisolating means formed at the bottom of the trench.

By the virtue of the above described structure, the number of trenchesformed on the surface of the semiconductor substrate can be reduced inassociation with the number of the corresponding photosensitive regions,whereby the degree of integration can be increased, the opening ratio ofthe photosensitive regions is improved, and the resolution of the solidstate image sensing device can be enhanced.

According to a further aspect of the present invention, a method formanufacturing a solid state image sensing device comprises the steps of:

a. selectively forming an insulating film for separating elements on asurface of a semiconductor substrate having a first conductivity type;

b. forming a first impurity region of a second conductivity type on thesurface of the semiconductor substrate;

c. forming a trench in a prescribed region of the surface of thesemiconductor substrate;

d. forming second impurity regions of the second conductivity type onboth side surfaces of the trench;

e. forming a third impurity region of the first conductivity type havinghigher impurity concentration between the main surface of thesemiconductor substrate and the second impurity regions;

f. forming a first insulating film on the main surface of thesemiconductor substrate and in the trench;

g. forming a polycrystalline silicon layer on the surface of the firstinsulating film and patterning the same into a prescribed pattern;

h. forming a second insulating film on the surface of the firstinsulating film and the polycrystalline silicon layer;

i. applying a resist on the surface of the polycrystalline silicon layerand patterning the same to expose the surface of the second insulatingfilm formed in the trench;

j. anisotropically etching the second insulating film with the resistserving as a mask to expose the surface of the polycrystalline siliconlayer formed on the bottom portion of the trench; and

k. etching the polycrystalline silicon layer using the resist and thesecond insulating film as masks.

Therefore, two charge transfer devices insulated and isolated from eachother can be easily manufactured in a trench.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective cross sectional view showing a structure of acharge transfer device in accordance with one aspect of the presentinvention;

FIG. 2 is a schematic diagram illustrating the operation of the chargetransfer device shown in FIG. 1;

FIG. 3 is a schematic plan view showing a horizontal structure of asolid state image sensing device in accordance with a second aspect ofthe present invention;

FIG. 4 is a perspective cross sectional view showing a cross sectionalstructure of the device of FIG. 3 taken from the line IV--IV of FIG. 3;

FIGS. 4A to 4H are cross sectional views showing the steps ofmanufacturing the solid state image sensing device shown in FIG. 4;

FIGS. 5A, 5B and 5C are cross sectional views illustrating the operationof the solid state image sensing device shown in FIGS. 3 and 4;

FIG. 6 is a cross sectional view showing another embodiment of the solidstate image sensing device in accordance with the second aspect of thepresent invention;

FIGS. 7A, 7B, 7C, 7D, 7E, 7F, 7G and 7H are cross sectional viewsshowing in this order the steps of manufacturing the solid state imagesensing device shown in FIG. 6;

FIG. 8 is a cross sectional view showing a third embodiment of the solidstate image sensing device in accordance with the second aspect of thepresent invention;

FIG. 9 is a cross sectional view showing a fourth embodiment;

FIG. 10 is a cross sectional view of the solid state sensing deviceshowing a fifth embodiment;

FIGS. 11A, 11b, 11C, 11D, 11E and 11F are cross sectional views showinganother method of manufacturing the solid state image sensing deviceshown in FIG. 4;

FIG. 12 is a perspective cross sectional view showing a cross sectionalstructure of a conventional charge transfer device;

FIG. 13 is a plan view showing a horizontal structure of the chargetransfer device shown in FIG. 12;

FIG. 14 is a plan view showing a horizontal structure of a conventionalsolid state image sensing device; and

FIG. 15 is a cross sectional view taken along the line I--I of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in the followingwith reference to the figures.

FIG. 1 is a perspective cross sectional view of a charge transfer deviceshowing one embodiment in accordance with a first aspect of the presentinvention. Referring to FIG. 1, a p type silicon substrate 1 comprises atrench 2 formed in the "vertical" direction of the main surface thereof.The trench 2 having vertical sidewall surfaces is formed elongate in thedirection of the main surface. n⁻ impurity regions 14, 14 are formed onboth side surfaces of the trench 2. An oxide film 15 for isolation isformed on a bottom portion of the trench 2 between the opposing n⁻impurity regions 14, 14. The oxide film 15 for isolation is formed byLOCOS (Local Oxidation of Silicon) method. A channel stopper 17 isformed below the oxide film 15 for isolation. The channel stopper 17 isprovided to avoid a mixture of signal charges caused by two n channelregions 14 and 14 being in contact with each other at the bottom portionof the trench 2.

Surface channel stop layers 16 which are p⁺ impurity regions having theimpurity concentration higher than the p type silicon substrate 1 areformed on the upper portions of the n⁻ impurity regions 14 and 14. Thesurface channel stop layers 16, 16 are provided for preventing thecharge packets in the n⁻ type impurity regions 14 being trapped to thesurface of the substrate to decrease the transfer efficiency inoperation. By the provision of the surface channel stop layers 16, aso-called buried channel CCD is provided.

Electrodes 4 formed of polysilicon are formed on the inner side surfacesof the trench 2 and the surface of the p type silicon substrate 1. Aplurality of electrodes 4a, 4b are arranged along the longitudinaldirection of the trench 2. Every other ones of the electrodes 4b havetheir side portions laid over the remaining ones of the electrodes 4a.

As described above, in the CCD of this embodiment, two charge transferregions electrically insulated and isolated from each other by an oxidefilm 15 for isolation formed on the bottom portion of the trench 2 areformed on both side surfaces of the trench 2. Consequently, the areaoccupied by the charge transfer regions of the surface of the siliconsubstrate 1 is considerably reduced, enabling higher degree ofintegration.

The operation of the CCD in accordance with the present embodiment willbe described with reference to FIG. 2. FIG. 2 schematically shows thestructure of the CCD shown in FIG. 1. The overlapping portions betweenthe electrodes 4a and 4b of FIG. 1 are omitted. FIG. 2 shows a state ofoperation of a three-phase CCD. A plurality of electrodes 4a, 4b arearranged along the longitudinal direction of the trench 2. A group ofthree electrodes 4a, 4b, 4c are connected to a three phase clock voltagesource φ1, φ2 and φ3 through electrode wirings. In operation, electrons,which are the transfer charges, are introduced into the charge transferregion 14 by any means, and a clock voltage is applied such that thevoltages of the electrodes 4a, 4b, 4c will be φ1>φ2>φ3. Then, theelectrons gather in a bottom portion of a potential well formed in thesilicon substrate at a position opposing to the φ1 electrode having thelowest potential due to the drift and thermal diffusion. Thereafter, aclock voltage is applied to the electrode 4a, 4b, 4c such that φ3>φ1>φ2.Consequently, the potential in the potential well formed in portion ofthe silicon substrate opposing the φ3 electrode become the lowest.Consequently, the electrons gather in the portion of the siliconsubstrate opposing the φ1 electrode are transferred to the portion ofthe silicon substrate opposing the φ3 electrode. Thereafter, a clockvoltage applied to the electrodes 4a, 4b, 4c such that φ2>φ3>φ1. Then,the electrons gather in the portion of the silicon substrate opposing φ3electrode are transferred to the bottom portion of the potential well ata position of the silicon substrate opposing the φ2 electrode. Byrepeating the above described operation, the electrons in the chargetransfer regions are transferred in the direction of the arrow of FIG.2.

A first embodiment of a solid state image sensing device in accordancewith a second aspect of the present invention will be described in thefollowing. FIG. 3 is a plan view schematically showing a horizontalstructure of the solid state image sensing device. Referring to FIG. 3,the solid state image sensing device comprises a plurality ofphotosensitive regions 8 (pixels) for generating image signals, verticaltransferring CCDs 18 for transferring the image signals received byphotosensitive regions 8 in the vertical direction, a vertical shiftregister 19 for generating signals for transferring image signals bymeans of the vertical transferring CCDs 18, and a horizontaltransferring CCD 20 for transferring the image signals transmitted bythe vertical transferring CCDs 18 in the horizontal direction. Thevertical transferring CCDs 18 are formed between every other lines ofthe photosensitive regions 8. In the conventional solid state imagesensing device, the lines of the photosensitive regions 8 and thevertical transferring CCDs 18 are arranged alternately. Therefore, thestructure of the first embodiment is capable of reducing the horizontalarea occupied by the vertical transferring CCDs 18 and increasing theopening ratio, as compared with the prior art shown in FIG. 14, forexample. In one embodiment, the opening ratio as high as 75% isrealized.

The principle of operation of the solid state image sensing device of afirst embodiment will be described with reference to FIG. 3. First,signal charges are stored in the photoelectric converting region 8 for aprescribed period. Thereafter, a voltage high enough to open the readinggate is applied to φV₁ and φV₂ of the vertical shift register 19,whereby the signal charges stored in the photoelectric converting region8 pass the reading gate altogether to flow in the vertical transferringCCDs 18. Thereafter, the voltage applied to φV₁ and φV₂ of the verticalshift register 19 is lowered so as to close the reading gate. At thesame time, the vertical CCDs 18 are operated by applying voltagesalternately to φV₁ and φV₂, so as to transfer the signal charges in thedirection of the arrow A. A line of vertical CCDs are capable ofsimultaneously transferring the signal charges in the photoelectricconverting region 8, 8 on both sides thereof. The signal charges reachedthe horizontal transferring CCD 20 are transferred in the direction ofthe arrow B by the clock operation of φH₁ and φH₂. Signal charges ofeach pixel are extracted by the transferring operation. By repeating theabove described operations, ever changing images can be obtained.

FIG. 4 is a perspective cross sectional view of a portion shown by theline IV--IV of FIG. 3, showing a schematic structure of the verticaltransferring CCD 18. The characteristic of the structure of the verticaltransferring CCD 18 in accordance with the present embodiment is thattwo vertical transferring CCDs 18, 18 independent from each other areformed on both side surface portions of a trench formed in asemiconductor substrate. More specifically, referring to FIG. 3 and FIG.4, one trench among trenches shown in FIG. 3 is formed parallel to eachother extending in the column direction on the surface of a p typesilicon substrate 1. n⁻ impurity regions 14, 14 which will be channelregions are formed on both side surfaces of the trench 2. n typeimpurity regions 11 are formed on the surface of the p type siliconsubstrate 1, which portions constitute the photosensitive regions 8. Thephotosensitive regions 8 are insulated and isolated from each other byan oxide film 21 for isolation. An oxide film 15 for isolation isprovided on a bottom portion of the trench 2 for isolating andinsulating the n.sup. - impurity regions 14, 14 from each other. Surfacechannel stop layers 16, 16 which are p⁺ impurity regions having higherconcentration are formed between the upper portion of the n⁻ impurityregions 14, 14 and the surface of the p type silicon substrate 1. Apotential barrier 22 which is a p impurity region having higherconcentration is formed surrounding the n⁻ impurity regions 14, 14.Electrodes 4a, 4b formed of silicon is formed on the inner surface ofthe trench 2 and the surface of the p type silicon substrate 1 with aninsulating film 3 interposed therebetween. A plurality of electrodes 4a,4b are arranged in the longitudinal direction of the trench 2corresponding to the photosensitive regions 8. A portion of each of thegate electrode 4a, 4b comprise a gate electrode 13 of a transfer gatefor reading signal charges from the photosensitive region 8.

The method of manufacturing the solid state image sensing device shownin FIG. 4 will be described in the following with reference to FIGS. 4Ato 4H.

Referring to FIG. 4A, a channel stopper 32 formed of a p type impurityregion having higher concentration than the substrate is formed on aprescribed region of a surface of a p type silicon substrate 1, and anoxide film 15 for isolating element is formed by the LOCOS method on thechannel stopper 32. Thereafter, n type impurities 24a are ion implantedto the surface of the p type silicon substrate 1 to form n type impurityregions 11 which will be the photosensitive regions 8.

Referring to FIG. 4B, the oxide film 15 for isolation on the surface ofthe p type silicon substrate 1 is selectively removed by etching.Thereafter, a trench 2 is formed by etching at a position where theoxide film 15 for isolation is removed, by using a resist pattern 40 asa mask. On this occasion, the channel stopper 32 remains on the upperportion of the trench 2, the remaining channel stopper 32 forming asurface channel stop layer 16 of the CCD.

Referring to FIG. 4C, p type impurity ions 25 are introduced to bothside surfaces of the trench 2 by oblique ion implantation, therebyforming potential barriers 22 formed of p⁺ impurity regions havinghigher concentration.

Referring to FIG. 4D, a silicon nitride film 41 is formed on the surfaceof the p type silicon substrate 1 and in the trench 2. The siliconnitride film 41 is patterned so as to expose the surface of the p typesilicon substrate 1 only in the trench 2.

Referring to FIG. 4E, the surface of the p type silicon substrate 1exposed at the bottom of the trench 2 is selectively oxidized by thermaloxidation using the silicon nitride film 41 as a mask. By this step, theoxide film 15 for isolation is formed on the bottom of the trench 2. Themethod of forming the oxide film for isolation at the bottom of thetrench is described in detail in U.S. Ser. No. 041,672.

Referring to FIG. 4F, the silicon nitride film 41 is removed andthereafter, n type impurity ions 24b are introduced to both sidesurfaces of the trench 2, again by oblique ion implantation. By this ionimplantation, the n⁻ impurity regions 14, 14 constituting n channel areformed.

Referring to FIG. 4G, a gate oxide film 3 is formed on the surface ofthe p type silicon substrate 1.

Referring to FIG. 4H, a polycrystalline silicon layer is formed on thesurface of the gate oxide film 3 and the polycrystalline silicon layeris patterned to have a prescribed shape. By doing so, the chargetransfer electrodes 4a and 4b are formed. Thereafter, the surface of thep type silicon substrate 1 is covered with an insulating film.

The method of oblique ion implantation to the sidewalls of the trenchemployed in the above described manufacturing process is disclosed indetail in, for example, "Depth Profiles of Boron Atoms with LargeTilt-Angle Implantations; G. Fuse et al., J.E.S: Solid-State Science andTechnology", 1986 Vol. 133. No. 5, p996. More specifically, the impurityions can be implanted to the entire surface of the trench sidewall bytilting the incident angle of the impurity ions.

The operation of the solid state image sensing device of the presentembodiment will be described in the following with reference to FIGS. 5Ato 5C. These figures illustrate the changes of the potential and signalcharges in the photosensitive region, the reading gate region and thecharge transferring portion. First, referring to FIG. 5A, signal charges23 are stored in the photosensitive region 8 corresponding to theincidental light. At this time, the reading gate is closed.

Referring to FIG. 5B, after charges are stored in the photosensitiveregion 8 for a prescribed time period, the reading gate is opened andsimultaneously, the signal charges stored in the photosensitive regionare read to the charge transferring portion.

Referring to FIG. 5C, the reading gate is again closed, and the signalcharges are transferred in a prescribed direction by means of a verticaltransferring CCD constituting the charge transferring portion.

By the repetition of the above described operation, the signal chargesof the line of pixels on both sides can be simultaneously read by twocharge transferring means formed in one trench.

A second embodiment in accordance with the second aspect of the presentinvention will be described with reference to FIG. 6. The secondembodiment shown in FIG. 6 is different from the structure of the solidstate image sensing device shown in FIG. 4 in that means for insulationand isolation formed on the bottom portion of the trench 2 has adifferent structure. Namely, in the second embodiment, the electrodes 4formed on the inner surfaces of the trench 2 are isolated from eachother at the bottom portion of the trench 2 and an insulating film isfilled therebetween. By this structure, the vertical transferring CCD 18formed on one side side surface of the trench 2 is electricallyinsulated and isolated from a vertical transferring CCD 18 formed onanother side surface of the trench.

By virtue of the above described structure, one vertical transferringCCD 18 can operate completely independent from the other verticaltransferring CCD 18. Such CCDs capable of independent operations can beeffectively utilized in a CCD memory and the like.

FIGS. 7A to 7H are cross sectional views showing the steps ofmanufacturing the solid state image sensing device in accordance withthe second embodiment. The method for manufacturing the solid stateimage sensing device shown in FIG. 6 will be described with reference tothese figures.

First, as shown in FIG. 7A, an oxide film 21 for isolating elements isformed on a prescribed region of a surface of a p type silicon substrate1 by using the LOCOS method or the like. Thereafter, an n type impurityregion 11 which will be the photosensitive region 8 is formed by ionimplantation of n type impurities to the surface of the p type siliconsubstrate 1.

Thereafter, as shown in FIG. 7B, a trench 2 is formed on a prescribedregion of the surface of the p type silicon substrate 1 using resistpatterns 33 as masks. The width of the trench is, for example, 1 μm.

Thereafter, as shown in FIG. 7C, n type impurity ions 24 are implantedon both side surfaces of the trench 2 by oblique ion implantation usingthe resist patterns 33 as masks, to form n⁻ impurity regions 14, 14which will be the n channel regions of the CCD. The depths of the n⁻impurity regions 14, 14 to the bottom portion can be controlled byadjusting the incidental angle of the impurity ions.

Thereafter, as shown in FIG. 7D, p type impurity regions 16 are formedby ion implantation of boron (B) 25, for example, near the intersectingportions of the upper portions of the trench 2 and the surface of the ptype silicon substrate 1 by oblique ion implantation, thereby providingsurface channel stop layers 16 which are the p⁺ impurity regions ofhigher concentration.

The incident angle of the boron ions 25 is selected such that the rangeof entrance of the boron ions 25 is regulated by the edge of the trench2. Therefore, the channel stop layer 16 is formed only on the upperportion of the trench 2.

Thereafter, as shown in FIG. 7E, an insulating film 3 such as a siliconoxide film is formed on the surface of the p type silicon substrate 1and on the inner surface of the trench 2 by thermal oxidation method.Thereafter, a polysilicon layer is formed on the surface of theinsulating film 3. The polysilicon layer is patterned to a prescribedshape to form n type electrode 4. Thereafter, a silicon oxide film 27 isformed on the substrate of the electrode 4 and on the surface of theinsulating film 3.

As shown in FIG. 7F, a resist 28 are applied on the surface of thesilicon oxide film 27, and the resist is patterned to expose only thesurface of the silicon oxide film 27 in the trench 2.

As shown in FIG. 7G, a portion of the silicon oxide film 27 positionedabove the bottom portion of the trench 2 is removed by anisotrophicetching, using the patterned resist 28 as a mask. But doing so, thesurface of the electrode 4 positioned on the bottom surface of thetrench is exposed.

Thereafter, as shown in FIG. 7H, the electrode 4 is anisotropicallyetched by using the resist 28 and the silicon oxide film 27 as masks, toselectively remove a portion of the electrode 4 positioned on the bottomsurface of the trench 2. By doing so, the electrode 4 on one side of thetrench 2 is separated from the electrode 4 on the other side surface ofthe trench.

Thereafter, the resists 28 are removed. Thus, the main portion of thesolid state image sensing device having the structure shown in FIG. 6 isprovided.

The steps shown in FIGS. 7A to 7D are also employed in manufacturing theabove described first embodiment and in manufacturing the structure ofother embodiments which will be described later.

A third embodiment of the solid state image sensing device in accordancewith the second aspect of the present invention will be described. FIG.8 is a cross sectional view corresponding to the cross sectionalstructure of the solid state image sensing device shown in FIG. 4. Thethird embodiment has a different insulating and isolating structureformed on the bottom portion of the trench 2. Namely, in the thirdembodiment, a SOG (Spin-on-Glass) layer 29 is formed in the bottomportion of the trench 2 in order to insulate and isolate n⁻ impurityregions 14, 14 formed on both side surfaces of the trench 2 from eachother. Since the SOG has a low viscosity, it can be easily buried in thebottom portion of a minute trench.

FIG. 9 is a cross sectional view showing a fourth embodiment in whichonly the structure for isolating elements on the bottom portion of thetrench is different, as in FIG. 8. In the fourth embodiment, apolysilicon layer 30 which is in an electrically floating state isformed in the bottom portion of the trench 2. The formation of channelbetween the n⁻ impurity regions 14, 14 can be prevented by this layer,so that the impurity regions are insulated and isolated from each other.

FIG. 10 is a cross sectional view showing a fifth embodiment of thesolid state image sensing device shown in FIG. 4. In the fifthembodiment, a p well region 31 which is a p⁺ impurity region havinghigher concentration is formed in the p type silicon substrate 1, andthe CCD is formed in the p well region 31. The p well region 31 has thesame function as the potential barrier 22 in the structure of FIG. 4.Namely, if the pn junction region of the photosensitive region 8 isflooded by the stored photoelectric charges, the p well region 31prevents the photoelectric charges from directly entering the n⁻impurity regions 14 of the CCD. The structure having the p well regioncan be incorporated with the element isolating structures shown in FIGS.6, 8 and 9.

Another aspect of the present invention will be hereinafter describedwith reference to FIGS. 11A to 11F, which is a manufacturing methoddifferent from that described with reference to FIGS. 7A to 7D inassociation with the structure of the solid state image sensing deviceshown in FIG. 4. First, as shown in FIG. 11A, a channel stopper 32 whichis a p type impurity region having higher concentration than thesubstrate is formed on a prescribed region of a surface of the p typesilicon substrate 1. An oxide film 15 for isolating elements is formedon the channel stopper 32 by the LOCOS method. Thereafter, n typeimpurities 24a are ion implanted to the surface of the p type siliconsubstrate 1 to form n type impurity regions 11 of the photosensitiveregions 8.

Thereafter, as shown in FIG. 11B, only a portion of the oxide film 15for isolation which is formed on a region on the surface of the p typesilicon substrate 1 on which the trench is to be formed is selectivelyremoved by etching. Thereafter, a trench 2 is formed by etching at aposition from which the oxide film 15 for isolation is removed. At thistime, the channel stoppers 32 are left on the upper portion of thetrench 2, which channel stoppers constitute the surface channel stoplayers 16 of the CCD.

Thereafter, as shown in FIG. 11C, p type impurity ions 25 are implantedby oblique ion implantation to both side surfaces of the trench 2,thereby forming a potential barrier 22 of p⁺ impurity region havinghigher concentration.

Thereafter, as shown in FIG. 11D, n type impurity ions 24 are implantedby oblique ion implantation on both side surfaces of the trench 2 toform n⁻ impurity regions 14, 14 which will be the n channels.

Thereafter, as shown in FIG. 11E, an insulating film 3 is formed on thesurface of the p type silicon substrate and on the inner surface of thetrench 2.

Thereafter, the same manufacturing steps as shown in FIGS. 7E to 7H arecarried out to provide the solid state image sensing device shown inFIG. 11F.

In the manufacturing method of the present embodiment, the surfacechannel stop layer 16 of the CCD is formed in the same step for formingthe element isolating structure for isolating the photosensitive region8 by using the channel stopper 32 formed below the oxide film 15 forisolation. By doing so, the surface channel stop layers 16 can be easilyprovided.

Various modifications of the solid state image sensing device shown inFIG. 4 have been proposed as described with reference to the second tosixth embodiments. More specifically, the structure of FIG. 4 is a basicstructure. A first group of embodiments is in association with thestructure for isolating elements formed on the bottom portion of thetrench 2. The structure for isolating the electrodes 4 shown in FIG. 6,the isolating structure employing the SOG layer 29 shown in FIG. 8 andthe isolating structure employing the polysilicon layer 30 shown in FIG.9 are included in this group. A second group of the modifications is inassociation with the structure for preventing leak of signal chargesfrom the photosensitive regions 8 to the n channel regions 14, 14. Thepotential barrier structure 22 shown in FIG. 4 and the structure of thep well regions 31 shown in FIG. 10 are included in this group. A thirdgroup of modifications is in association with the manufacturing method.The method for forming the trench 2 and n channel regions 14, 14 shownin FIGS. 7A to 7D and the similar manufacturing steps shown in FIGS. 11Ato 11F are included in this group. The first, second and third groups ofmodifications may be arbitrarily combined with each other.

The insulating and isolating means formed on the bottom portion of thetrench 2 can be also applied to the CCD shown in FIG. 1.

As described above, in the charge coupled device in accordance with thepresent invention, independent charge transfer devices are formed onboth side surfaces of a trench formed on the surface of thesemiconductor substrate, which devices are insulated and isolated fromeach other at the bottom portions of the trench, whereby the occupiedarea of the surface of the semiconductor substrate can be reduced, andthe degree of integration of the device can be increased. In addition,in the solid state image sensing device employing such charge coupleddevices, the vertical charge transferring regions can be arranged onevery other lines of photosensitive regions, whereby the surface areaoccupied by the charge transfer regions can be reduced, the openingratio can be improved, and the sensitivity of the solid state imagesensing device can be improved.

In addition, in accordance with the manufacturing method of the presentinvention, charge transfer regions can be easily formed on sidewalls ofa minute trench, which regions are independent from each other as theyare insulated and isolated from each other at the bottom portion of thetrench.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

What is claimed is:
 1. A method for manufacturing a solid state imagesensing device, comprising the steps of:selectively forming aninsulation film for isolating elements on a surface of a semiconductorsubstrate having a first conductivity type; forming a first impurityregion of a second conductivity type on the surface of saidsemiconductor substrate; forming a trench on a prescribed region of thesurface of said semiconductor substrate; forming second impurity regionsof the second conductivity type on both side surfaces of said trench;forming a first insulating film on the main surface of saidsemiconductor substrate and in said trench; forming a polycrystallinesilicon layer on the surface of said first insulating film andpatterning the same into a prescribed pattern; forming a secondinsulating film on said first insulating film and on the surface of saidpolycrystalline silicon layer; applying a resist on the surface of saidpolycrystalline silicon layer and patterning the same to expose thesurface of said second insulating film formed in said trench;anisotropically etching said second insulating film by using said resistas a mask to expose the surface of said polycrystalline silicon layerformed on the bottom surface of said trench; and etching saidpolycrystalline silicon layer by using said resist and said secondinsulating film as masks.
 2. A method of manufacturing a solid stateimage sensing device according to claim 1, further comprising, after thestep of forming the second impurity region in said trench, the stepofforming, by oblique ions implantation, third impurity regions of thefirst conductivity having higher concentration between said secondimpurity regions and main surface of said semiconductor substrate.
 3. Ina solid state image sensing device having charge coupled devices onsidewall portions of a trench, a method of forming said trenchcomprising the following steps of:forming a first impurity region of thefirst conductivity type having higher concentration on a prescribedregion on a surface of the semiconductor substrate; forming aninsulating film for isolating elements on a surface of said firstimpurity region; introducing impurities to the surface of saidsemiconductor substrate to form a second impurity region removing saidinsulating film for isolating elements at a prescribed position; andforming a trench in the first impurity region with said insulating filmfor isolating elements removed therefrom.